The present invention relates to a pressure filter for treating liquor suspensions of a chemical pulp mill.
When producing chemical pulp by the sulfate method, spent cooking liquor, e.g. black liquor, is evaporated and combusted, and the obtained residual, e.g. soda smelt, is dissolved in water to form green liquor that contains sodium carbonate and sodium sulfide. Green liquor contains insoluble substances, such as metal oxides, which are removed to avoid being enriched in the chemical cycle. The separation of green liquor dregs containing the non-process elements from green liquor typically takes place in a clarifier or a filter. The green liquor dregs are thickened and washed usually in a dregs filter. The cleaned green liquor is causticized into new cooking liquor. In the causticizing, the sodium carbonate in the green liquor reacts with burned lime (CaO).
White liquor containing sodium hydroxide and calcium carbonate (lime mud) thus obtained are separated and the white liquor is circulated to pulp cooking. The separation of white liquor and lime mud can be performed either in a clarifier or by means of filtration. White liquor filters are typically usual sock (candle) filters or disc filters that are either pressure or vacuum filters. The lime mud is washed in order to remove alkali. The lime mud is thickened and washed usually in a lime mud filter for combustion in a calcination apparatus, such as a lime kiln, wherein the lime mud is regenerated to burnt lime (calcium oxide) to be returned back to causticizing.
A pressure filter that has long been used at causticizing plants, such as described above, typically includes tubular filtering elements. These pressure filters have been used for washing lime mud and filtration of white liquor and green liquor. The filtering elements typically include a sock filter.
The liquor suspension, such as lime milk, is pumped into a pressure vessel, wherein the liquor travels through tubular filtering elements into a filtrate chamber, while the lime mud remains on the filtering surface. When the thickness of the lime mud cake on the surface is adequate (e.g. after 3-5 minutes), the liquor filtrate is made to flow to the opposite direction through the filtering elements (e.g. for 1-10 seconds) for releasing the lime mud cake from the filtering surface. After a short while (typically 30 seconds) the lime mud begins to descend towards the bottom of the filtration vessel, and a new filtering cycle may begin. Lime mud is continuously removed from the bottom of the filtration vessel into a washing stage, which usually is a dilution wash, and the lime mud can be separated by means of a similar kind of filter. The remaining lime mud is returned into the feed container for the filter.
In practice, solid substances accumulate on the filtering surface of the filtering element, which are not released during back flushing. This impairs the flow of the filtrate through the filtering surface of the element, which is seen in the pressure increase in the lower part of the filter. Also, the elements may get clogged. To prevent clogging, the filtering elements can be washed.
Washing is conventionally accomplished by means of water and acid washing. An acid mixture in an acid container is circulated through the filter, from up down-wards through the filtering elements. The acid used in the wash is neutralized with sodium hydroxide and led into a weak liquor container. After being washed with the acid mixture, the filter is flushed with water. The filter may also be washed with water prior to acid washing.
During the circulation of the acid mixture, precipitates may flow with the acid mixture and enter the interior of the filtering element. The precipitates should not be on the interior surface of the filtering element. A further problem noticed in washing operations and back-flushing of the filtrate (cake release) is that the washing liquid or filtrate flowing into the interior of the filtering element from up is not distributed evenly, but its pressure is directed to a greater extent to the lower part of the element. Thus, precipitate cake remains on the outer surface of the upper part of the filtering element.